1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus used for a laser printer, a digital copying machine, and a plain-paper facsimile machine.
2. Description of the Related Art
Electro-photographic image forming apparatuses used for laser printers, digital copying machines and plain-paper facsimile machines have recently been developed from the view points of colorization and speedup, and specifically, tandem-compatible image forming apparatuses having a plurality of (usually, four) photoconductors have now come into widespread use. Although color electrophotographic image forming apparatuses can be of a system that has a sole photoconductor and rotates the photoconductor a number of turns which is equal to the number of colors (for example with four colors and one drum, it is necessary to turn the drum four turns), productivity is inferior.
However, in the case of a tandem system, an increase in the number of light sources is unavoidable, thereby causing a color drift due to a difference in the wavelengths between the plurality of light sources and a rise of cost due to an increase in the number of components.
In addition, deterioration in a semiconductor laser has been mentioned as a cause of a writing unit failure. When the number of light sources is increased, the failure probability is increased, and recyclability is deteriorated.
There is an example contrived not to increase the number of light sources in a tandem system, for example as disclosed in Japanese Unexamined Patent Application No. 2002-23085. In this example, by use of a pyramidal mirror or a flat mirror, beams from a common light source scan different scanning surfaces. However, by this method, although the number of light sources can be reduced, the number of deflecting mirror faces is limited to two at maximum, and thus, the problem of speeding up still remains.
In order to solve the problem, the present invention has been proposed to use polygon mirrors overlapped in two tiers with the phases shifted from each other as a means for scanning different scanning surfaces by beams from a common light source. There is a conventional art having a configuration similar to that of the present invention as disclosed, for example, in Japanese Unexamined Patent Application No. 2001-83452.
However, the ultimate purpose of this conventional art is to increase the scanning width, but not for scanning different scanning surface.